Presently, there are two leading technologies involved in image generation in electronic projectors: digital mirror device (DMD) and liquid crystal device (LCD). The present invention relates to the DMD technology.
Typically, a DMD chip is made of an array of bi-stable micromirrors, which may be repositioned between two stable positions. The angle between the planes of the mirrors in the two stable positions is called “the deflection angle”. The position of each mirror can be independently switched by electronic control. The projector is designed in such a manner that in one position (the “on” position) the light from the mirror is directed towards the projection lens. In the other (“off”) position, the light is dumped (directed away from the lens, and ideally absorbed). By controlling the positions of the mirrors it is possible to create a binary image. Gray levels are generated by vibrating the mirrors during the video frame between the two positions in a predetermined pattern/duty cycle. The amount of light reaching the screen from a given micromirror is proportional to the duty factor of the “on” position.
One of the main quality parameters in an image projector is the light output. The light throughput of an optical device is limited theoretically by a quantity called “étendue”. When the light passes through several components in series, the component with the smallest étendue becomes the limiting factor of the entire system. Generally, the price of an optical component increases with its étendue. Therefore, the most expensive component is usually the limiting factor in the light output. In electronic projectors, these are the image generating devices, which are either DMD's or LCD's.
The étendue E of an optical component is given by E=A·Ω, Equation (1), where A is the component entrance pupil area, and Ω is the component solid acceptance angle. For small angles, we can approximate Ω=θ2, where θ is the planar acceptance angle, so thatE=A·θ2  Equation (2)
The étendue of a DMD chip is determined by its area and by the mirror deflection angle. The price of the DMD chip increases with its area, and, therefore, there is a constant market pressure to reduce the chip size. This leads to a decrease in étendue. The deflection angle is mainly limited by technological constraints. Currently, DMD micromirror arrays are available with a deflection angle of 20°. A DMD chip with a deflection angle of 24° is under development. This is sought in order to allow size (and price) reduction, without reducing the chip étendue.
In U.S. Pat. No. 6,382,799 (Nishikawa et al.), incorporated herein by reference, an attempt was made to increase the étendue of a DMD chip by way of providing greater lateral separation between beams reflected off the DMD chip. There was disclosed a projection optical apparatus has a light source for emitting light, an illumination optical system for emitting as illumination light the light radiated from the light source, a Digital Micromirror Device (DMD), having a plurality of micromirrors, a total internal reflection prism composed of a first prism for totally reflecting and thereby directing the illumination light exiting from the illumination optical system to the DMD and a second prism for transmitting the signal light reflected from the DMD, and a projection optical system for projecting the signal light transmitted through the second prism onto a projection surface. The prism assembly is introduced here to optically increase the lateral distance between the deflected beams off each micromirror, without having to increase the distance of the DMD chip from the lens, thus effectively increasing the étendue of the system.
An object of the present invention is to provide a novel design for a DMD chip in order to increase DMD chip étendue without increasing its area or deflection angle. This will allow projector designs with increased light output without increase in DMD cost. Such development will improve the position of DMD based projectors in competition with LCD based projectors.